The invention relates to a tool and a method used in electrochemical machining. More particularly, the invention relates to a tool and method for forming non-circular holes from pre-formed circular holes by selectively removing insulating material on the surface of the tool.
A specialized adaptation of electrochemical machining, known as shaped-tube electrochemical machining (STEM), is used for drilling small, deep holes in electrically conductive materials. STEM is a noncontact electrochemical drilling process which can produce holes with aspect ratios as high as 300:1. It is the only known method which is capable of manufacturing the small, deep holes used for cooling blades of efficient gas turbines.
The efficiency of a gas turbine engine is directly proportional to the temperature of turbine gases channeled from the combustor of the engine and flowing over the turbine blades. For example, for gas turbine engines having relatively large blades, turbine gas temperatures approaching 1500° C. (2700° F.) are typical. To withstand such high temperatures, these large blades are manufactured from advanced materials and typically include state-of-the-art type cooling features.
A turbine blade is typically cooled using a coolant such as compressor discharge air. The blade typically includes a cooling hole through which the air passes. A further design advancement has been the addition of internal ridges in the cooling hole to effect turbulent flow through the hole and increase cooling efficiency. Cooling features within the hole such as turbulence promoting ribs, or turbulators, thus increase the efficiency of the turbine.
The cooling holes commonly have an aspect ratio, or depth to diameter ratio, as large as 300:1, with a diameter as small as a few millimeters. The turbulators extend from sidewalls of the hole into the air passage about 0.2 millimeters (mm), for example.
One method currently used for drilling the cooling holes in turbine blades is a shaped-tube electrochemical machining (STEM) process. In this process, an electrically conductive workpiece is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes function as cathodes in the electrochemical machining process, while the workpiece acts as the anode. As the workpiece is flooded with an electrolyte solution from the drilling tubes, material is deplated from the workpiece in the vicinity of the leading edge of the drilling tubes to form substantially straight-walled (circular) holes.
Turbulated ridges are formed in the cooling holes by a modification of the standard shaped-tube electrochemical machining (STEM) process for drilling substantially straight-walled (circular) holes. One common method is termed cyclic dwelling. With this technique, the drilling tube is first fed forward, and then the advance is slowed or stopped in a cyclic manner. The dwelling of the tool which occurs when the feed rate is decreased or stopped creates a local enlargement of the hole diameter, or a bulb. The cyclic dwelling causes ridges to be formed between axially spaced bulbs. Cyclical voltage changes may be required. These ridges are the turbulators.
The cyclic dwelling method is very low in process efficiency compared to shaped-tube electrochemical machining (STEM) drilling of substantially straight-walled holes because of the long time required for drilling each bulb individually by cyclic tool dwelling. The dwell time required to form a single bulb can be greater than the time for drilling an entire substantially straight-walled hole.
To alleviate the problems associated with the cyclic dwelling method using the STEM method, another method known as pulsed electrochemical machining (PECM) positions an electrode comprising a hollow electrically conductive cylinder coated with an electrically insulating coating in a pattern into the substantially straight-walled hole. The pattern on the electrode is a series of rings that defines raised areas or ridges to be machined in the substantially straight-walled hole. The exposed conductive material on the surface of the electrode defines areas where bulbs are formed by removal of metal from the wall of the pre-formed hole. The raised areas or ridges are created in the wall of the pre-formed hole where no deplating occurs in the vicinity of the insulated portions of the surface of the electrode. These ridges are the turbulators.
However, all the above methods are directed to producing raised areas or ridges, which are the turbulators, in a pre-formed, substantially straight-walled (circular) hole. None of the above methods are directed to producing a non-circular hole from the pre-formed, substantially straight-walled (circular) hole. Accordingly, there is a need in the art for a new and improved method for manufacturing a non-circular hole from a pre-formed, substantially straight-walled hole.